In the field of space-based imaging, optical sensors such as visible/infrared IR/microwave wavelength sensors and radiometers typically require routine calibration while in operation to ensure proper scaling of data returned from the sensor. Payload for a launch vehicle is an engineering concern for any space-based venture, so the calibration mechanisms generally are designed to be small in size and weight.
One known space based imaging system is termed the Moderate Resolution Imaging Spectroradiometer MODIS. Another is termed Visible/Infrared Imager/Radiometer Suite VIIRS, but MODIS is explained herein for background. MODIS measures the earth's land, oceans, and atmosphere from NASA's TERRA satellite, which circles along a polar orbit. Every one to two days, MODIS views the earth's surface in its entirety, collecting data in thirty six different spectral bands ranging from visible to long range IR. The breadth of wavelength imaging and of data acquired requires precise calibration of the MODIS sensing equipment, for which a solar diffuser SD is used for calibration in the visible light range.
Solar diffusers typically use high reflectance Lambertian plates by which to provide a diffuse source of known luminance to the imaging sensors for calibration. MODIS uses a space grade, Spectralon® material as its solar diffuser, a proprietary thermoplastic formulation of Labsphere, Inc. of North Sutton, N.H. Over time, the reflectance of these plates change due to a number of factors, most of which are related to exposure of the plate to solar irradiance. Once the reflectance of the reference plate changes, calibration relies on knowing the extent of that change. The MODIS sensors scan the reference plate, providing a known illumination as an input to the sensors from which they may be calibrated. To maintain calibration precision, changes in the plate's reflectance must be measured and quantified as it degrades over time from to solar irradiance.
FIG. 1A is prior art schematic diagram of the MODIS calibration system 10. The solar diffuser 11 receives direct sunlight 12 and reflects it into a reflected input port 13 of a Solar Diffuser Stability Monitor SDSM 15 along a first reflected path 12a, and to the sensors (not shown) along a second reflected path 12b. The SDSM 15 compares the reflected beam received at the reflected input port 13 to direct sunlight received at a direct input port 19 to determine true reflectance of the SD 11. When the sensor scans along the second path 12b and views the SD 11, the input to the sensor is known from the sunlight intensity and the measured reflectance of the SD 11.
FIG. 1B shows an orbital path 19 of a vehicle such as that carrying MODIS 10 superimposed over a plan view of the earth. The MODIS sensors rotate 360° about an axis substantially aligned with the orbital path 19. The lines across which the one MODIS sensor scans, traced by the sensor optical axis, are shown partially as reference numbers 20a, the thin lines that cross the orbital path near the perpendicular. Note that the illustrated scan lines 20a are abbreviated; the actual MODIS sensors scan continuously and only scan the earth's surface during a portion of each 360° rotation, so the illustrated scan lines represent only one sensor. When facing the earth, the sensors collect data. When facing away from the earth and into the space-based vehicle, the MODIS sensor scans across the solar diffuser along the second path 12b (FIG. 1A). Because the sensors scan the SD 11 on each 360° rotation (about every 2-3 minutes), the SD 11 is continually exposed to sunlight (excepting those times when sunlight does not align with the SD 11).
FIG. 1C illustrates the same orbital path 19, but shows the scan track of what is commonly termed a “whisk broom” sensor. Rather than rotate 360° as in MODIS, the whisk broom sensors always align their optical axis with the earth's surface (or other target) but scan from side to side to a limited extent in order to collect data from the periphery of the orbital track 19. The track scanned by a whisk broom type sensor is shown in FIG. 1C as a sine-type track 20b crossing the orbital path 19, and may be confined close to the track 19 or vary widely from it. Of course, the sensor may scan along a similar path 20b offset from the actual orbital path 19 rather than superimposed over it, depending upon how the sensor is mounted in the orbiting vehicle. Similar to the whisk broom type sensor is a “push broom” type sensor, which does not scan side to side but rather looks always at the target (earth's surface) along the orbital track or at a fixed offset therefrom.
Unlike the rotating MODIS sensors that change their field of view to take in the SD 11, calibration is performed on both whisk broom and push broom type sensors by physically interposing the SD between the sensor and its target. While calibration is performed much less frequently on whisk or push broom sensors as compared to MODIS or other rotational type sensors (typically spanning weeks or months as opposed to MODIS' every few minutes), a risk arises should the latter SD apparatus malfunction while positioned between the sensor and the target. Malfunctioning in that position causes the push broom type sensor to be unable to see beyond the SD, and render it unusable for practical purposes. A similar malfunction with a whisk broom type sensor may still allow the scanning whisk broom sensor to scan aside the malfunctioning SD for a portion of its scan, but the data it collects would be largely reduced in volume. Risk of a malfunctioning SD is to be minimized, as space-based repairs are difficult, costly, and often not practically viable.
What is needed in the art is a calibration system for a sensing system that relies on movement of the solar diffuser that provides a more robust design capable of error free operation over a substantial period of time. The invention is particularly well suited for sensor systems that require solar diffusers that are interposed between the sensor and the target for calibration, such as whisk or push broom type sensors.